It is common to adopt an open-loop operating manner in a linear drive circuit of a load such as an LED load, and in order to prevent an overhigh input power when the line voltage is slightly higher, a line voltage compensation circuit is often considered to be added. In one embodiment shown in FIG. 1, a line voltage sampling circuit constituted by a resistor RD and current mirrors M0 and MOSFET M1 samples the voltage at a DRAIN side (namely, a voltage drop obtained by subtracting an LED load from the line voltage VBUS) of a power tube and outputs current, since an output current of the line voltage sampling circuit is in direct proportion to the voltage at a DRAIN side of the MOSFET Mpwr, a resistor Rcmp regulates the baseline voltage VREF based on a current output by the sampling circuit, and a drive voltage VREF′ is output to an operational amplifier Amp to drive the MOSFET Mpwr, and a source voltage VCS of the MOSFET Mpwr follows the drive voltage VREF′, in this way, the LED load current is equal to VCS/Rcs.
The technical solution provided by FIG. 1 adopts a line voltage compensation technology with a single slope, and the technical solution has the shortcoming that the LED load's current line regulation is contradictory to the input voltage's line regulation, namely, the drive voltage VREF′ has an inversely proportional linear relationship with the voltage at a DRAIN side of the MOSFET Mpwr, as shown in FIG. 2, in other words, if better input voltage's line regulation is desired, the LED load's current line regulation will be poorer.